1. Field of the Invention
The present invention relates to a light guide plate and a liquid crystal display device having the same, more particularly, in which the light guide plate has a function of a diffusing sheet and a prism sheet to remarkably improve brightness of light refracted from the light guide plate toward the liquid crystal panel within a front visual angle, and the number of parts is diminished to reduce thickness of the device and save manufacturing costs.
2. Description of the Related Art
In general, a liquid crystal display device requires a separate light source since a display itself cannot emit light on its own. The light source can be disposed behind the display or at a side thereof.
The light source, when disposed at a side of the display, decreases an overall thickness of the device, thus typically applied to notebook computers or mobile phones having a thin display surface.
FIG. 1 is a configuration view illustrating a conventional liquid crystal display device. A light source 10 such as a fluorescent lamp or a light emitting diode is disposed at a side of a light guide plate 20 in a length direction to emit light generated to a liquid crystal panel 50.
A reflective sheet 25 is disposed under the light guide plate 20 to reflect light toward the liquid crystal panel 50. Additionally, a diffusing sheet 30 is disposed over the light guide plate 20 to diffuse, in several directions, light which is reflected onto the reflective sheet 25, refracted to the light guide plate 20 and exits to the liquid crystal panel 50.
Also, prism sheets 41 and 42 are interposed between the diffusing sheet and the liquid crystal panel 50 to collect light propagating through the diffusing sheet 30 within a front visual angle.
FIG. 2 is a graph illustrating brightness distribution of light propagating through each part of a conventional liquid crystal display device. Brightness distribution of light passing through the light guide plate 20 is indicated with a line ‘d1.’ Here, brightness is shown to be high at a visual angle of 50° to 70°. On the other hand, brightness is relatively greatly degraded in a front visual angle of 0°.
Moreover, in a case where the diffusing sheet 30 is adopted, brightness distribution of light passing through the light guide plate 20 is indicated with a line ‘c1.’ Here, the brightness distribution that is biased to one side is shifted toward a front visual angle. In a case where the prism sheets 41 and 42 are disposed in addition to the diffusing sheet 30, brightness distribution of light passing therethrough is completely shifted to a front visual angle as indicated with lines ‘b1’ and ‘a1.’ Thus, the brightness distribution is found to be increased at a front angle of ±30°.
As demonstrated by the graph, the light guide plate in the conventional liquid crystal display device has a drawback in brightness distribution that is biased to one side from a front visual angle. To overcome this drawback and achieve a display with a desired image, the diffusing sheet 30 and prism sheets 41 and 42 should be additionally provided. This, however, increases thickness and volume of the liquid crystal display device.
Also, the diffusing sheet 30, and the prism sheets 41 and 42 account for a significant proportion of costs in the liquid crystal display device. This mainly leads to increase in overall manufacturing costs of the liquid crystal display device.
Meanwhile, in a method to enhance directionality and gathering of light refracted from the light guide plate 20, microstructures may be formed on the light guide plate 20 to uniformly refract light generated from the light source 10 toward the light crystal panel. The microstructures can be formed by metal etching, lens formation process or mechanical machining.
Korean Patent Application Publication No. 2003-12568 (published on Feb. 12, 2003) discloses hemispherical or pyramidal elements formed on an underside surface of the light guide plate.
FIG. 3 is a graph illustrating brightness distribution of light propagating through a light guide plate having diffusing elements formed thereon. The brightness distribution of light propagating through the light guide plate with the hemispherical and pyramidal elements thereon is biased to one side (right side in FIG. 3) as indicated with a line ‘d2’ and a line ‘b2.’
Meanwhile, in a case where a diffusing sheet 30, a first prism sheet 41 and a second prism sheet 42 having an array of prisms crossing those of the first prism sheet 41 are additionally disposed over the light guide plate 20 with the hemispherical elements thereon, brightness distribution of light is indicated with a line ‘c2.’ Moreover, in a case where the diffusing sheet 30 and the horizontal prism sheet 42 are additionally disposed over the light guide plate 20 with the pyramidal elements thereon, brightness distribution of light is indicated with a line ‘a2’, which is substantially similar to that of ‘c2.’
Accordingly, the pyramidal elements, when arrayed on the light guide plate yield more advantageous effects than the hemispherical elements. That is, the pyramidal elements adopted require relatively a fewer number of prism sheets than the hemispherical elements, while ensuring substantially identical brightness at a substantially identical front visual angle.
Furthermore, the pyramidal elements which assure better brightness at a front visual angle than the hemispherical elements as just described, are improved in terms of their array configuration thereby to attain higher and more uniform brightness distribution.
FIGS. 4 (a) and (b) are plan views illustrating various configurations of pyramidal diffusing elements arrayed on a light guide plate. As shown in FIG. 4 (a), the diffusing elements 21 are arrayed on the light guide plate 20 along imaginary lines T connecting vertexes of the pyramidal elements 21, in parallel to a reference line O connecting a central point of a light source 10 to a central point of the light guide plate 20. In the diffusing elements, two edges facing the imaginary lines T are set in parallel with the reference line O. This means that the diffusing elements are rotated 0° about the reference line O. Alternatively, as shown in FIG. 4 (b), each of the diffusing elements 21 is rotated 45° clockwise or counterclockwise about an axis extending through a vertex of the diffusing element perpendicularly to a surface of the body so that an array of the diffusing elements is angled 45° about the reference line O.
FIG. 5 is a graph illustrating brightness distribution of light propagating through light guide plates having pyramidal diffusing elements arrayed at a rotation angle of 0° and pyramidal diffusing elements arrayed at a rotation angle of 45° and hemispherical diffusing elements arrayed thereon, respectively. As shown, a line ‘a3’ indicates brightness distribution of light passing through the light guide plate having the pyramidal diffusing elements formed at a rotation angle of 0°. A line ‘b3’ indicates brightness distribution of light passing through the light guide plate having the pyramidal diffusing elements arrayed at a rotation angle of 45°. A line ‘c3’ indicates brightness distribution of light passing through the light guide plate with the hemispherical diffusing elements thereon, and a diffusing sheet, first and second prism sheets disposed over the light guide plate.
As demonstrated by the graph, to attain the same brightness at a front visual angle as in a case where the light guide plate with hemispherical diffusing elements thereon, and the diffusing sheet and prism sheet are adopted, the light guide plate with the pyramidal diffusing elements thereon also requires the diffusing sheet and prism sheet to be disposed thereover. This increases thickness and weight of an overall device and drives up manufacturing costs.